1. Field of the Invention
The present invention relates to an optical scanner using an image forming mirror instead of an f.theta. lens in a second image forming system.
2. Description of the Related Art
In various kinds of general known optical scanners relative to an optical printer, etc., a light beam deflected at an equal angular velocity is converged as a light spot on a scanned face to scan the scanned face. An f.theta. lens is generally known as an optical system for converging the deflected light beam as the light spot on the scanned face and performing an optical scanning operation using the light spot at an equal speed. However, an image forming mirror system having a linearity correcting function is recently used instead of the f.theta. lens as a proposal. For example, such a proposal is shown in Japanese Patent Application Laying Open (KOKAI) No. 1-0200221.
In a recent optical scanner, a high density recording is required to improve the quality of a written image. To realize this high density recording, it is necessary that no diameter of a light spot for scanning the scanned face is greatly changed as the height of an image is increased. The diameter of the light spot is influenced by loci of an image forming point in a main scan-corresponding direction and a cross scan-corresponding direction, i.e., field curvatures in these directions in an image forming system for converging the deflected light beam as a light spot on the scanned face. The main scan-corresponding direction is set to a direction corresponding and parallel to a main scanning direction on a virtual optical path on which an optical path from a light source to the scanned face is linearly developed. The cross scan-corresponding direction is set to a direction corresponding and parallel to a cross scanning direction on this virtual optical path.
It is sufficient to set the field curvatures of the image forming system to zero in both the main scan-corresponding direction and the cross scan-corresponding direction so as to restrain the change in diameter of the light spot. However, it is practically impossible to set the field curvatures of the image forming system to zero. In reality, the field curvatures are considerably caused in the main scan-corresponding direction and the cross scan-corresponding direction. The change in diameter of the light spot caused by the field curvature in the main scan-corresponding direction can be corrected by electrically controlling a time for writing one dot to such an extent that no practical problems about this change are caused. Accordingly, allowance is relatively high with respect to the field curvature in the main scan-corresponding direction.
However, the change in diameter of the light spot in the cross scan-corresponding direction cannot be electrically corrected in this way. Accordingly, when the field curvature in the cross scan-corresponding direction is large, the diameter of the light spot in the cross scan-corresponding direction is greatly changed in accordance with the image height. Therefore, in general, the above image forming system is designed such that the field curvature in the cross scan-corresponding direction is reduced as much as possible. However, when the image forming system is really manufactured, an error in manufacture of the image forming system is inevitably caused to a certain extent. Further, when the image forming system is assembled into a body of the optical scanner, an error in assembly of the image forming system is inevitably caused. When there are such errors in manufacture and assembly of the image forming system, an image forming position of the deflected light beam in each of the main and cross scan-corresponding directions is shifted from a designed position. Therefore, the diameter of the light spot formed on the scanned face in each of the main and cross scan-corresponding directions is different from a designed diameter.
The error in diameter of the light spot based on the errors in manufacture and assembly of the image forming system can be corrected by electrically controlling the above time for writing one dot with respect to the main scan-corresponding direction. However, it is difficult to electrically correct the error in diameter of the light spot with respect to the cross scan-corresponding direction.
Further, when there is a so-called inclination of a deflecting face in an optical deflector for deflecting the light beam at an equal angular velocity, the position of a main scanning line scanned by the light spot is changed in the cross scanning direction so that a so-called jitter is caused. The above problems about the change in diameter of the light spot and the generation of jitter are also caused when the above image forming mirror system is used as an image forming system.